1. Field of the Invention
This invention relates to compounds which are isolated from the oral secretions of beet armyworm caterpillars, to chemically synthesized compounds, and derivatives thereof. These compounds induce plants to produce and/or release a blend of volatile compounds which, for example, can attract and/or retain beneficial insects, increase fragrance in flowers, increase essential oils in plants, such as mint, which are grown for agricultural production, increase production of pharmacologically important compounds such as taxol, etc, in plants. The present invention also relates to methods for making the compounds and to their use as effective inducers of useful plant compounds.
2. Description of the Related Art
Arthropod plant pests cause extensive and severe damage to major agricultural commodities, both in the field and in the greenhouse environment. In addition to feeding damage, many of these insects also transmit viral diseases. Chemical insecticides are used to control insects that damage agricultural commodities such as corn, cotton, barley, beans, citrus, etc. However, recent concerns about insecticide residues on commodities, resistance of insects to chemical insecticides, hazardous exposure to people who apply pesticides, environmental contamination, destruction of natural biocontrol agents such as beneficial insects, and lack of newly developed insecticides have increased the need for alternative control methods. Furthermore, as pests become more resistant to pesticides, more frequent treatments are required which compounds the problems identified above. An alternative to chemical pesticides is the use of biocontrol agents such as beneficial insects which parasitize and/or eat harmful insects. A useful way to attract and/or keep beneficial insects on crops is needed in order to control harmful herbivorous insects and to at least reduce the amount of pesticides needed for the control of herbivores.
Stress-induced reactions in plants are common and the production and transport of chemicals in response to such stress are proposed to function as a direct defense against herbivores and pathogens. For example, terpenoids and indole are likely to be defensive compounds or by-products of stress-produced compounds. However, in addition to initiating direct chemical defenses, herbivore-injured plants benefit directly by signaling information into their environment. This phenomenon appears to be common; several of the terpenoids released by damaged corn seedlings are also released by leaves of other plants under attack by caterpillars or mites. The chemical signals, in addition to being attractants to natural enemies of the herbivores, may also function as repellents to herbivores. Plants contain a number of organic chemicals which attract pollinating insects or symbiotic organisms and provide self-defense against pathogens and herbivores (Blechert et al, Proc. Natl. Sci, USA, Volume 92, 4099-4105, May, 1995). Wound damage to the leaves of plants from a number of families results in the synthesis of proteinase inhibitor proteins at the wound sites as well as in distal leaves. Jasmonic acid and methyl jasmonate are examples of two plant-derived chemicals which can regulate the expression of wound-inducible proteinase inhibitor genes. Farmer and Ryan (Plant Cell, Volume 4, 129-134, 1992; Trends Cell Biol., Volume 2, 236-241, 1992; both herein incorporated by reference) show that the signal involved in the induction of the high molecular weight proteins involves a lipid-based transduction system which yields jasmonic acid and its methyl ester. It is believed that plant wounding by insects or microbial pathogen attack leads to an interaction of elicitors with receptors thus initiating the octadecanoic-based pathway from C.sub.18 fatty acid linolenic acid to jasmonic acid. Synthetic jasmonic acid also acts as a powerful inducer of de novo defense protein synthesis, simulating a wound response.
Jasmonic acid is a naturally occurring compound identified in plants from at least nine families (Farmer et al, Trends in Cell Biol., supra). Its methyl ester has been used as a perfume fragrance. Jasmonic acid and methyl jasmonate exhibit diverse biological activities in plants in regulating physiological processes and gene expression. Exogenously applied jasmonates have been found to accelerate senescence, induce the synthesis of vegetative storage proteins (VSPs) and proteinase inhibitor proteins in leaves, inhibit pollen germination, stimulate ethylene production in tomato fruit, and promote chlorophyll degradation in barley and anthocyanin production in soybeans. Jasmonates are cyclopentanone compounds (See FIG. 2a) which are commonly present throughout the plant kingdom. The structures and bioactivities of jasmonates are thoroughly reviewed by Hamberg and Gardner (Biochimica et Biophysica, Volume 1165, 1-18, 1992) and Parthier (Biochimica et Biophysica Acta, Volume 104, 446-454, 1991), the contents of each of which are herein incorporated by reference herein. Jasmonates have been described as exerting a wide range of differing effects on virtually all plants. These effects range from inhibition to promotion of plant processes. As Parthier describes, the effect exhibited on the plant may even be concentration dependent, with some processes stimulated at lower concentrations but inhibited at higher concentrations. Jasmonates have been reported to regulate growth patterns in soybean, induce gene expression during zygote embryogenesis in Brassica, cause tendril coiling in Bryonia and cell-cycle-dependent disruption of microtubles in tobacco cells. Jasmonates have also been shown to inhibit seed germination and seedling growth, stimulate seed germination (at lower concentrations), promote seed dormancy breaking, and promote leaf senescence. Farmer et al (1992, supra) also show that octadecanoid precursors of jasmonic acid; linolenic acid, 13(s)-hydroperoxylinolenic acid and phytodienoic acid can also act as signals for proteinase inhibitor induction in tomato leaves when applied to leaf surfaces. Tazaki (Japanese kokai 2-92220 (A), published Apr. 3, 1990, patent application no. 63-242432, filed Sep. 29, 1988), Yoshihara et al,Agric. Biol. Chem., Volume 53, 2835-2837, 1989), Matsuki et al (Biosci. Biotech, Biochem., Volume 56, 1329-1330, 1992) and Koda et al (Plant Cell Physiol., Volume 29, 969-974, 1988) all disclose treating potato stem fragments with jasmonates in culture to induce tuber formation. These compounds are considered to be endogenous regulators of plant responses.
Corn plants attacked by caterpillars release volatile terpenoids. The parasitic wasp Cotesia marginiventris, a generalist larval parasitoid that attacks many different lepidopterous species, locates caterpillar hosts in response to terpenoid release. In locating the caterpillar, the wasp injects an egg into it. In response the caterpillar immediately slows its feeding habit. Later, the wasp egg develops into a larvae that feeds internally on the caterpillar. Tumlinson et al (C&EN, Sep. 7, 1992) show that the oral secretions from caterpillars, which contain a combination of digestive fluids and saliva, induce local terpenoid release when applied to scratched surfaces of leaves on corn plant seedlings. The plant response cannot be induced by artificial damage unless the damaged sites are treated with the regurgitant of larvae (Florida Entomologist, Volume 74(1), 42-50, March, 1991). The terpenoids are released from the scratched leaves as well as from undamaged leaves. This indicates that the release of terpenoids is systemic and not just localized to the damaged leaves. Undamaged plants are far less attractive to beneficial insects than plants damaged overnight by beet armyworm (BAW, Spodoptera exigua Hubner) larvae. The strongest attraction is observed when larvae are put back on an already damaged corn seedling. Furthermore, the corn seedlings on release of the terpenoids and indole become less palatable to the BAW larvae.
While it is known that oral secretions from herbivores and plant-derived precursors of jasmonic acid induce the release of terpenoids by plants, other factors, especially herbivore-derived, have not been identified, isolated, and/or synthesized. Currently, the compounds of the present invention are only available by using crude preparations of oral secretions of herbivorous insects. It is desirable to be able to chemically synthesize these compounds and active derivatives in order to make their use economically feasible. The only substance from insect oral secretions found to induce volatile chemical release by plants thus far is beta-glycosidase from Peris brassicae larvae(Mattiacci et al, PNAS, USA, Volume 92, 2036, 1995). The present invention provides non-enzymatic herbivore-derived compounds, synthetic compounds and derivatives, different from related art compounds. The compounds of the invention induce plants to produce and/or emit volatile compounds, such as for example, compounds which attract natural enemies of herbivores, increase production of pharmacologically important compounds, increase fragrance of flowers, increase production of plant essential oils, etc.